1. Field of the Invention
The present invention relates to an electrode arrangement for electrical stimulation of biological material, having at least one stimulation electrode via which the biological material can be fed a stimulus signal, and having at least one counter electrode which forms a counter pole to the stimulation electrode.
The invention relates furthermore to a multi-electrode array for use in such an electrode arrangement.
2. Related Prior Art
An electrode arrangement of the said type and a corresponding multi-electrode array are disclosed, for example, in U.S. Pat. No. 4,628,933.
That document describes a so-called retina implant, that is to say an implant which is to be inserted in the region of the retina of an eye. The retina implant has a multiplicity of stimulation electrodes arranged in the manner of a field and with the aid of which artificially generated stimulus signals are fed to specific somatic cells located in the retina. The stimulus signals are generated with the aid of light-sensitive elements, of which the retina implant likewise has a multiplicity. The implant described is intended to restore at least a certain ability to see to people who have lost their ability to see, for example as a consequence of a disease known as retinitis pigmentosa.
After implantation, the stimulation electrodes of the described retina implant are in direct contact with the surrounding cell tissue and with body fluids which are present in the region of the cell tissue. It is known that so-called Helmholtz double layers form at the interface between the electrodes and the cell tissue or the body fluid, as happens generally at any phase transition between a metal electrode and an electrolytic liquid. The reason for this layer is the different type of charge transport in the said materials. The Helmholtz double layer constitutes in a first approximation an electric capacitor which is recharged upon stimulation of the cell tissue. This effect is denoted in the skilled community by the term electrode polarization, inter alia. A voltage which is denoted below as polarization voltage is then present across the charged electrode capacitor.
If the polarization voltage exceeds a certain threshold value, undesired redox reactions occur which can lead to irreversible tissue damage. Various methods are known in the prior art for the purpose of avoiding this.
It is proposed in the said U.S. Pat. No. 4,628,933 to arrange between the stimulation electrodes and the at least one counter electrode (denoted as ground conductor there) a large resistor via which the polarization voltage can be reduced. The coating of the electrodes with barium titanate or iridium is proposed as a further solution, since these materials are intended to have only a slight tendency to polarization. Moreover, the electric stimulation is intended to be performed with a rectangular alternating signal whose mean value is zero so that the interface capacitor is always discharged again.
In the German book entitled “Die Bedeutung der Phasengrenze zwischen alloplastischen Festkörpern und biologischen Geweben für die Elektrostimulation” [“The importance of the interface between alloplastic solids and biological tissues for electrostimulation”] by Armin Bolz, published by Fachverlag Schiele und Schön, 1994, it is proposed, with reference to the said problem, for the stimulation electrode and the counter electrode to be cyclically short-circuited with the aid of a so-called autoshort switch, in order to achieve a quick charge reduction at the interface (loc. cit. page 49).
A further approach to avoiding undesired reactions consists in dimensioning the capacitance of the phase transition, by the configuration of the electrode surfaces, to be as large as possible in order to minimize the polarization voltage. Moreover, parameters of the stimulus signal such as, in particular, the pulse durations and pulse amplitudes, can be dimensioned so as to exclude undesired reactions even under worst-case conditions. Finally, it is possible in principle to calculate the polarization voltage from the pulse durations and pulse amplitudes of the stimulus signal in order to control the stimulation on the basis of the results.
None of the known methods is optimum, however. Precisely the last-mentioned measures have the disadvantage that transient processes are above all detected, whereas no account is taken of a static electrode polarization rising in the course of time. Consequently, it is necessary in practice to take account of safety reserves in the dimensioning, and this renders optimum stimulation of the cell tissue difficult.
The regular or continuous discharge of the interface capacitor impairs the degrees of freedom in the temporal dimensioning of the stimulus signals. Likewise, the use of zero-point symmetrical alternating signals constrains the freedom of configuration. In addition, some of the measures are certainly well suited for excluding undesired reactions in the region of one or a few electrodes. However, if a multiplicity of electrodes, that is to say a multi-electrode array, is used for the stimulation of the cell tissue such as, for example, in the case of a retina implant, real time monitoring for the avoidance of undesired reactions is very complicated.